The present disclosure relates to power charging, and more specifically, to wireless power receivers that wirelessly receive power from wireless power transmitters and wireless power transmitters that wirelessly transmit power to wireless power receivers.
A mobile terminal, such as a portable phone or a PDA (Personal Digital Assistant), uses rechargeable battery. In order to charge such a battery, electric energy is supplied by a separate charging device that plugs into the mobile device, or otherwise mates the contact terminals of the mobile device to contact terminals of the charging device. However, this type of charging method exposes the contact terminals on the mobile device and/or the charging device to the environment. Accordingly, the contact terminals may get contaminated by foreign matter, thereby interfering with charging the battery. Additionally, the exposed contact terminals on the mobile device may make it harder to make the mobile device water resistant.
Wireless charging, or contactless charging, technology has been developed and used for a number of electronic devices. Such wireless charging technology uses wireless power transmission and reception. The wireless charging technology allows a battery to be charged by merely putting a mobile device, such as a cell phone, on a charging pad without connecting the portable phone to a separate charging device. Wireless charging technology is used for many devices currently, including for wireless electric toothbrushes and wireless electric shavers. It is expected that wireless charging technology will advance significantly as electric cars become more common.
Presently, wireless charging technology main interest is with the inductive coupling method (inductive method), the resonance inductive coupling method (resonance method), and the RF/microwave radiation method. When power is transferred by the inductive coupling method, referred to in this disclosure as the inductive method, current in a primary coil generates a magnetic field, and that magnetic field induces current in a secondary coil. Power transmission using inductive coupling has excellent energy transmission efficiency. However, the primary and secondary coils must be very close to each other for efficient energy transfer. Coils may also be referred to as inductors.
The resonance inductive coupling method, referred to in this disclosure as the resonance method, is a type of inductive coupling method where both the transmitter and the receiver have circuits tuned to a specific frequency. Professor Soljacic at MIT demonstrated this wireless charging system in 2005 by transferring power to an electronic device several meters away using Coupled Mode Theory. The resonance method uses the concept of resonance frequency, where resonance frequency is a characteristic of all objects. An object may preferentially generate or receive energy at its resonance frequency. For example, when a tuning fork is struck, it will vibrate at its resonance frequency. A wine glass near the turning fork with the same resonance frequency will absorb the acoustic energy of the vibrations generated by the tuning fork until the wine glass shatters. Similarly, a power transmitter using the resonance method generates a magnetic field of a specific frequency. Energy is efficiently transferred via that magnetic field when there is a receiving device with receiving circuitry that has that resonance frequency. Due to larger distances between the transmitting device and the receiving device, the resonance method may have lower energy transmission efficiency than the inductive method.
In some cases wireless charging may provide increased charging efficiency over outlet charging. Various techniques are being researched to further raise the charging efficiency. Merely increasing power from the wireless power transmitter may lead to a sharp heat build-up in the wireless power receiver due to larger electric current in the coils or capacitors.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.